1. Field of the Invention
The present invention relates to a motor and, more particularly, to a brushless DC motor.
2. Description of the Related Art
A conventional brushless DC motor disclosed in U.S. Pat. No. 5,217,351 titled as “Small Fan” is well known. The disclosed brushless DC motor includes a housing and an impeller. The housing has a base providing an axial tube and holding two coils, with the axial tube receiving a bearing inside and the coils being in plate shapes. The impeller has a bowl-shaped hub, a plurality of blades arranged on an outer surface of the hub with equal intervals and expanding in radial directions about the hub, a shaft extending between the hub and the base with two ends of the shaft respectively coupling with a center point inside the hub and the bearing of the axial tube, and an annular magnet also in a plate shape combined inside the hub. The annular magnet can thereby be driven by alternating magnetic fields created by the coils when currents flow through the coils. Thus, the impeller revolves.
However, because the hub and blades of said conventional brushless DC motor are generally produced by injection molding, reduction in axial size of the hub is limited, and the size of the conventional brushless DC motor is, therefore, impossible to be further decreased.
In order to provide a brushless DC motor with smaller axial size, another conventional brushless DC motor is proposed, which is disclosed in a Taiwan patent application number 94140896, titled as “Thin Fan” and shown in FIGS. 1 and 2. Referring to FIG. 1, the conventional DC motor comprises a base 51, a shaft 52, a blade member 53, a coupling member 54, and a magnet-plate 55. The base 51 provides a plurality of coils 511. The two ended shaft 52 mounts on the base 51 with one end and mounts on the coupling member 54 with the other end. The coupling member 54 has a radial protrusion 541 for the blade member 53 to be firmly sandwiched between the radial protrusion 541 and the magnet-plate 55. Now referring to FIG. 2, there is a gap, indicated as “G”, between the magnet-plate 55 and the coils 511 to let flux generated by the magnet-plate 55 pass through the coils 511 for flux linkage. Therefore, when currents flow through the coils 511 for the coils 511 to generate opposite magnetic field directions, the magnet-plate 55 and the blade member 53 are turned. By said structure of the conventional brushless DC motor, the size of the DC motor is decreased.
However, in operation of the DC motor, a part of the flux generated by the magnet-plate 55, which is identified as leak flux and indicated as “L”, does not pass through the coils 511. Also said leak flux “L” and the coils 511 may induce high-frequency resonance easily, which can further induce high-frequency noise. Accordingly, there is a need for redesigning the conventional brushless DC motor.